The present invention relates to a compound layered type of sensing device used for multiple measurement, and in particular, to such a sensing device which can be disposed in the exhaust gas system of an automobile internal combustion engine in order to detect the concentration of NOx contained in its exhaust gas, an air fuel ratio and other characteristics.
There has been known a gas sensor disposed in the exhaust system of an automobile internal combustion engine and configured to detect not only the concentration of NOx contained in its exhaust gas but also the concentration of oxygen contained in the exhaust gas in the form of a xcex-characteristic. This gas sensor is very effective, because physical quantities, such as the concentration of NOx, an A/F (air fuel ratio), and a xcex-characteristic can be detected by the same single sensor.
As a compound layered type of sensing device used for such a gas sensor, a four-sell type of device has been known conventionally, of which sectional configuration is shown in FIG. 1. As shown therein, this device 9 has a porous substrate 91, solid electrolyte plate 92, spacer 93, substrate 94, spacer 95, solid electrolyte plate 96, spacer 97, and layered type of heater 99, which are stacked in turn.
The sensing device 9 has a first chamber 81 and a second chamber 82, in addition to a reference gas chamber 89 in which a reference gas is charged. The first chamber 81 communicates with the outer space of the device 9 through a first diffusive resistance passage 810, while the second chamber 82 communicates with the first chamber 81 through a second diffusive resistance passage 820.
Additionally, the sensing device 9 has a first electrochemical cell 83, located therein so as to be exposed to the first chamber 81, that is capable of pumping oxygen from the outer space depending on an applied voltage thereto. The sensing device 9 also has a second electrochemical cell 85, located therein so as to be exposed to the second chamber 82, that is capable of outputting current corresponding to the concentration of NOx contained in an exhaust gas in response to a specified voltage applied thereto.
The sensing device 9 has also a third electrochemical cell 84 formed so that it is able to measure the concentration of an exhaust gas exhausted outside the device. Also, in the sending device 9 is provided a fourth electrochemical cell 86 that is exposed to the second chamber 82 and has the function of monitoring the concentration of oxygen present in the second chamber 82.
An electromotive force provided by the fourth electrochemical cell 86 is used to control voltage to be applied to the first electrochemical cell 83.
This type of sensing device makes it possible to measure the concentration of oxygen contained in an exhaust gas together with the concentration of NOx contained in an exhaust gas, so that a xcex-characteristic and air fuel ratio can be obtained. Since the single sensor enables parallel measurement of both concentration of NOx and xcex-characteristic (or air fuel ratio), providing the device with saved space and reducing a fitting cost of the device.
However, the foregoing compound layered type of sensing device has a drawback of being fairly low in accuracy of measurement of oxygen concentration, whereby being also low in accuracy of measurement of the xcex-characteristic.
This drawback is mainly results because the first and third electrochemical cells 83 and 84 are located on the same solid electrolyte plate. The voltage applied to the first electrochemical cell 83 fluctuates and its output is fairly larger in amount than that of the third electrochemical cell 84, which largely influences the output of the third electrochemical cell 84.
As understood from FIG. 1, a total of five plates ranging from the first solid electrolyte plate 92 to the second one 96 are present between two electrodes of the second electrochemical cell 85. This presence gives rise to a larger amount of inner resistance to the second electrochemical cell, making the second electrochemical cell liable to influences of surrounding conditions such as temperature.
Therefore, an object of the present invention is to solve the foregoing drawback owned by the conventional sensing device, and to provide a compound layered type of sensing device capable of measuring the xcex-characteristic with precision.
Another object of the present invention is to provide a compound layered type of sensing device capable of measuring the xcex-characteristic with precision by detecting oxygen concentration more accurately.
Still another object of the present invention is to provide a compound layered type of sensing device, which is directed to multiple detection purposes, capable of concurrently detecting various physical parameters relating to a gas to be measured, with influences of surrounding conditions suppressed.
In order to realize the object, as one aspect of the present invention, there is provided a compound layered type of sensing device, comprising: a plurality of solid electrolyte plates; and first to third electrochemical cells each having a single pair of electrodes disposed on the solid electrolyte plates, in which a concentration of a gas specified from a gas to be measured pre-processed based on oxygen pumping by the first electrochemical cell is detected by the second electrochemical cell and a difference in electromotive force between the gas to be measured and a reference gas is detected by the third electrochemical cell; wherein the single pair of electrodes of the third electrochemical cell is disposed on a same surface of one of the solid electrolyte plates, and both of the first and third electrochemical cells are located with different ones of the solid electrolyte plates.
In this configuration, the single pair of electrodes of the third electrochemical cell is placed on the same surface of the same solid electrolyte plate with the electrodes close to each other. Thus the inner resistance of the third electrochemical cell becomes smaller, thereby resisting influence of sensor output measuring devices and/or outer circuits. Further, the smaller inner resistance reduces influence of outside atmospheric temperature.
Further, disposing the one pair of electrodes of the third electrochemical cell in that way makes it possible that voltage applied to this cell is fixed at a constant magnitude, without changes over time. Outputs of the third electrochemical cell are therefore reluctant to fluctuations over time.
Because each of the first and third electrochemical cells is disposed with different solid electrolyte plates, the third electrochemical cell shows resistance to influence of temporal changes in voltage applied to the first electrochemical cell.
Hence, accuracy of detecting differences in electromotive force between the gas to be measured and the reference gas by the third electrochemical cell can be raised. The difference in electromotive force is proportional to the concentration of oxygen contained in the gas to be measured. When the compound layered type of seasoning device according to the present invention is disposed in the combustion exhaust system of an internal combustion engine, a xcex-characteristic of the engine can be measure with precision the third electrochemical cell.
Additionally, the applied voltage to the first electrochemical cell may be controlled in such a manner that a map is produced in advance based on amounts of current obtained from the concentration of oxygen contained in a gas to be measured, and the applied voltage is controlled using the map. Higher accuracy is therefore secured in controlling the concentration of oxygen present in the chambers into a predetermined constant.
Thus a compound layered type of sensing device with higher accuracy of measurement of the xcex-characteristic can be provided.
In the foregoing one aspect of the present invention, it is preferred that the device further comprises first and second chambers formed in the device and into which the gas to be measured is introduced, and a fourth electrochemical cell configured to detect the concentration of the oxygen present in at least one of the first and second chambers.
Hence, the fourth electrochemical cell is used for detecting the concentration of oxygen present in the first or second chamber and controlling the applied voltage to the first electrochemical cell. A gas to be measured can be measured in concentration with higher precision. Alternatively, correcting measurements acquired by the fourth electrochemical cell enables an objective gas to be measured in concentration with precision.
In this configuration, when the first and fourth electrochemical cells are made from inert electrodes (for example, made of Pt or Au) to a gas to be measured (NOx) and predetermined voltage is applied to both cells, the concentration of only oxygen present in the chambers can be measured. This measurement may be replaced by a technique of using electromotive force obtained from oxygen and a reference gas in the chambers.
Still, it is preferred that the first chamber communicates with an outside of the device via a first diffusive resistance passage and the second chamber communicates with the first chamber via a second diffusive resistance passage, the first electrochemical cell being configured, with one surface thereof exposed to the first chamber, so as to take oxygen in and out to and from the first chamber correspondingly to voltage applied to the first electrochemical cell, and the second electrochemical cell being configured, with one surface thereof exposed to the second chamber, so as to detect current corresponding to the concentration of the specified gas contained in the gas to be measured by applying a predetermined voltage to the second electrochemical cell.
According this configuration, the first electrochemical cell is able to control the concentration of oxygen existing in both of the first and second chambers to a constant amount. Fluctuations in oxygen concentration at both chambers cause disturbances to a gas to be measured. Hence, the control to a constant concentration enables measurement accuracy to be improved.
Accordingly, amounts of current corresponding to the concentration of a specified gas detected by the second electrochemical cell can be avoided from being influenced by the concentration of oxygen contained in a gas to be measured, even if the oxygen concentration fluctuates.
The first and second diffusive resistance passage can be formed with a porous member combined with a tiny through hole such as a pin hole.
Preferably, the device further comprises a plurality of reference chambers, wherein both of the second and fourth electrochemical cells are disposed to a same one of the reference gas chambers and either one of the first and second chambers.
This configuration permits the concentration of oxygen remaining within the first and second chambers to be measured accurately.
Still preferably, the device comprises a plurality of reference chambers, wherein each of the first and third electrochemical cells is disposed to a different one of the reference gas chambers.
Thus, without being affected by electric noise from the first electrochemical cell, the third electrochemical cell is able to perfume higher accurate measurement.
It is also preferred that an alumina-made plate intervenes between the first and second electrochemical cells so that both the cells are insulated to each other.
The first electrochemical cell is therefore insulated from the second electrochemical cell, whereby avoiding the second electrochemical cell from being influenced by voltage fluctuations or the like of the first electrochemical cell.
As another aspect of the present invention, there is provided a compound layered type of sensing device, comprising: first and second chambers into which a gas to be measured is introduced, the first chamber communicating with an outside of the device via a first diffusive resistance passage and the second chamber communicating with the first chamber via a second diffusive resistance passage; first and second reference gas chambers into which a reference gas is introduced; a first electrochemical cell, disposed to be exposed to the first chamber, for pumping oxygen correspondingly to voltage applied to the first electrochemical chamber; a second electrochemical cell, disposed to be exposed to the second chamber, for detecting current corresponding to a concentration of a gas specified in the gas to be measured by applying a predetermined voltage to the second electrochemical chamber; a third electrochemical cell for measuring a concentration of oxygen contained in the gas to be detected; and first and second solid electrolyte plates between which the first and second chambers are formed, wherein the first reference gas chamber is disposed on a surface of the first solid electrolyte plate facing the outside of the device and the second reference gas chamber is disposed to be exposed to the first and second chambers by way of the second solid electrolyte plate; the first electrochemical cell, disposed with the second solid electrolyte plate, has a pumping electrode exposed to the first chamber and a reference pumping electrode exposed to the second reference gas chamber; the second electrochemical cell, disposed with the first solid electrolyte plate, has a sensing electrode exposed to the second chamber and a reference sensing electrode exposed to the first reference gas chamber; and the third electrochemical cell, disposed with the first solid electrolyte plate, has an oxygen sensing electrode facing the outside of the device and a reference oxygen sensing electrode exposed to the first reference gas chamber, both of the oxygen sensing electrode and the reference oxygen sensing electrode being disposed on a same surface of the first solid electrolyte plate.
According to this aspect of the invention, the oxygen sensing and reference oxygen sensing electrodes of the third electrochemical cell are close to each other in distance. The inner resistance of the cell is therefore small. The third electrochemical cell shows resistance to influence of outer circuits and sensor output measuring apparatuses, thereby making a contribution to measurement with higher accuracy.
As stated before, a smaller-value of inner resistance of the third electrochemical cell reduces influence of outer atmospheric temperature, whereby leading to measurement of oxygen concentration with higher precision.
Further, it is the second electrochemical cell that is disposed with the first solid electrolyte plate with which the third electrochemical cell is disposed as well. Because the voltage applied to the second electrochemical cell is constant over time, the measurement accuracy of the third electrochemical cell has not so much influence from the second electrochemical cell.
Still further, the first electrochemical cell is located with the second solid electrolyte plate located with both first chamber and first solid electrolyte plate layered thereon. The third electrochemical cell has little influence of the fluctuating voltage applied to the first electrochemical cell.
This enables higher accuracy of measurement of the third electrochemical cell, whereby increasing the measurement accuracy of the xcex-characteristic.
Therefore, it is possible for the present invention to provide a compound layered type of sensing device superior in the measurement accuracy of the xcex-characteristic.